1. Field of the Invention
The invention relates to a method of manufacturing a vehicle body side structure and a vehicle body side structure.
2. Description of the Related Art
A center pillar is disposed in the vertical direction of a vehicle body side portion, as a vehicle body side structure. The center pillar is configured so as to have a pillar reinforcement between a pillar outer panel and a pillar inner panel.
In this vehicle body side structure, a strength change point is provided below the center portion of the center pillar in the vertical direction so that the strength of the member is higher in the upper portion from this strength change point and is lower in the lower portion.
This vehicle body side structure has an advantage. Since the lower portion of the center pillar deforms more easily than the upper portion, when a vehicle crashes at its side, the deformation shape of the center pillar is controlled and the center pillar is prevented from hurting an occupant, thereby protecting the occupant.
In more detail, the pillar reinforcement of the center pillar is formed by superposing the upper member and the lower member partially and joining these superposed portions each other by spot welding etc. The upper member is made of a high tensile steel sheet formed by hot press forming (hot stamping). The hot press forming is to press a heated steel sheet by a mold. Since the quenching effect by the mold enhances the annealing of the steel sheet, the strength of the steel sheet is increased significantly.
On the other hand, the lower member is made of an ordinary rolled steel sheet etc, and the strength is set lower than that of the hot press member. In other words, when the upper member and the lower member are compared, the upper member has higher strength than the lower member.
Next, referring to FIGS. 3 and 4, a conventional method of manufacturing a pillar reinforcement 7 will be described. The pillar reinforcement 7 includes an upper member 3a, a lower member 3c, and a reinforcement member 3b for reinforcing the upper member 3a. The reinforcement member 3b is provided at a height corresponding to the breast of an occupant.
As shown in FIG. 3, rolled steel members 1a, 1b and 1c are cut to have predetermined planar shapes using press machines 2a, 2b and 2c, respectively, to form the upper member 3a, the reinforcement member 3b and the lower member 3c. The steel members 1a and 1b are steel sheets for hot press, and the steel member 1c is a steel sheet for cold press.
The upper member 3a and the reinforcement member 3b are then heated to a predetermined temperature and hot press formed using press machines 4a and 4b. By this, the upper member 3a and the reinforcement member 3b are annealed by the quenching effect by the mold, and thus the strength is increased significantly.
On the other hand, since the lower member 3c is cold press formed by a press machine 4c without a heating process, the strength is lower than that of the upper member 3a and the reinforcement member 3b. In this case, the upper member 3a is bent at the peripheral end portion to form a ship bottom shape, and the reinforcement member 3b and the lower member 3c have such a shape that the peripheral end portion is bent corresponding to the shape of the upper member 3a. 
Then, as shown in FIG. 4, using a laser beam, laser trimming is performed so as to remove unnecessary portions of the upper member 3a, the reinforcement member 3b and the lower member 3c, and holes 5a and 5c for attaching necessary components are also formed.
Then the upper member 3a, the reinforcement member 3b and the lower member 3c are assembled. At this time, the reinforcement member 3b is superposed on the upper member 3a. Furthermore, the upper member 3a and the lower member 3c are partially superposed. Then the superposed portions of the upper member 3a and the reinforcement member 3b and the superposed portions 6 of the upper member 3a and the lower member 3c are spot welded to join the individual members, completing the pillar reinforcement 7. A relevant technique is disclosed in Japanese Patent Application Publication No. 2010-173403.
However, in the conventional method of manufacturing the pillar reinforcement 7, since the individual members are assembled and the spot welding is performed after the upper member 3a, the reinforcement member 3b and the lower member 3c are press formed as described above, there is a problem that the spot welding portion is limited. This is because a spot welding portion is limited to a flat region of each of the members and the flat region is narrowed unavoidably since the peripheral ends of the individual members are bent in the case of spot welding after press forming.
Furthermore, in the case of spot welding after press forming, since the spot welded portion and the peripheral portion thereof are naturally cooled after the welding, there is a problem that the strength is decreased.
Furthermore, since the upper member 3a, the reinforcement member 3b and the lower member 3c are hot press formed or cold press formed as described above, there is a problem that the process is complex.